3D Printing Model Correction
Perhaps Multiple Problems.
Edit Mode.  Select Non-Manifold identifies some problems. 
See Arrow in image above.
Add faces to holes you see.



Strange face. Edit Mode Face Selection. See arrow. Remove doubles can correct partially. Image above. 

Look for more problems.  This does not strive to be a tutorial.

Consider using Blender Mesh [Join]. Inspect each time with the menu choices of the first image.

Consider converting all n-gons to quadrangles or triangles. Quads and Tris.

Consider the Remesh Modifier. There is an Apply button.

Consider duplicating the mesh and making corrections to improve to the point when your 3D printer software has zero warnings.  I do not have your 3D printing software.